#app {
    width: 90%;
    height: 100%;
    background-color: red;
}
.allbox{
    width: 1200px;
    height: 200px ;
    background-color: rgb(18, 241, 241);
    margin: 0 auto;
    color: rgb(189, 59, 124);
}
.left,.right{
    width: 300px;
    height: 100%;
    background-color: blue;
    line-height: 200px;
    text-align: center;
}
.left{
    float: left;
}
.right{
    float: right;
}
.center{
    width: auto;
    height: 100%;
    background-color: pink;
    margin-left: 300px;
    margin-right: 300px;
    line-height: 200px;
    text-align: center;
}